The use of various compounds as markers or taggants for liquid and solid materials is well known. Fluorescent dyes have been used in many applications, the fluorescence characteristics of a sample of the marked material being used to determine the presence and concentration of the taggant in the material. Other known taggants include biological compounds, especially DNA and oligonucleotides, and also phosphors. A typical application of these taggants is in the tagging of liquids such as hydrocarbon fuels in order to identify the liquid at a subsequent point in the supply chain. This may be done for operational reasons, e.g. to assist in distinguishing one grade of fuel from another, or for other reasons, in particular to ensure fuel quality, deter and detect adulteration and to provide a means to check that the correct tax has been paid. Apart from fuels, many other products, such as pharmaceuticals, agrochemicals, cosmetics, perfumes for example, may be marked to identify a product produced at a particular source, which may be certified to a particular standard or branded as an original high value product or for other purposes.
The problem of providing taggants which are difficult to reproduce by unauthorised persons yet readily identifiable and quantifiable by authorised persons has been addressed in many ways. Some prior methods involve the separation of the marker compound from the liquid by means of extraction into a polar liquid or onto a solid absorbent. For example, U.S. Pat. No. 5,358,873 describes and claims a method of detecting gasoline adulteration by tagging with a rhodamine dye and then shaking a small sample of the suspected fuel in a vial containing a small quantity of un-bonded flash chromatography-grade silica. The presence of the rhodamine marker dye in the suspect sample colours the silica red. U.S. Pat. No. 4,659,676, U.S. Pat. No. 2,392,620, and U.S. Pat. No. 4,735, 631 describe other methods for fuel marking and analysis.
DNA has been described for use as a taggant for various products, however the quantitative detection of nucleic acids, for example using hybridisation or quantitative PCR methods, is not sufficiently reproducible to encourage its use as a marker for products where detection of dilution or adulteration by detection of relatively small differences in the concentration of the taggant is required.
SERS (Surface Enhanced Raman Spectroscopy) is an analytical method in which the surfaces of certain metals enhance the Raman spectrum of compounds adsorbed onto or located in close proximity to such surfaces. The effect is sometimes referred to as a plasmonic effect and the surfaces may be referred to as plasmonic surfaces. Well-known plasmonic materials include gold, silver and copper. Compounds which exhibit such an enhancement in their Raman signal are referred to as “SERS-active” compounds. The enhancement of the Raman spectrum may be used to detect SERS-active compounds when they are present at concentrations at which they would not be detectable by other methods, for example non-enhanced Raman spectroscopy. The plasmonic effect is increased when a SERS-active compound is close to more than one surface. For that reason, plasmonic surfaces tend to be rough or regularly contoured. Alternatively when SERS is carried out using colloidal nanoparticles of plasmonic materials, the plasmonic effect, and therefore the SERS enhancement, is increased by aggregation of the colloidal particles so that molecules of the SERS-active compound may benefit from proximity to two or more particles. When we refer to SERS in the present specification we intend to include other forms of surface enhanced spectroscopy (SES) such as SERRS (surface-enhanced resonance Raman spectroscopy). For brevity these methods will all be referred to as SERS.
WO2008/019161 describes a method of fuel identification with surface enhanced Raman spectroscopy (SERS) tags. This method includes the association of a substance having a known Raman spectrum with a quantity of fuel. In one embodiment, a nanoparticle including a SERS active core may be mixed into a fuel supply. In an alternative embodiment, a SERS active dye including a Raman active reporter molecule may be mixed with a quantity of fuel. If the quantity of fuel is tagged with a Raman-active dye, the process of identifying the quantity of fuel may include mixing into a sample of the fuel a colloid of Raman enhancing metal particles and then acquiring the Raman spectrum of the Raman active reporter molecule associated with the tag. Suitable metals include, but are not limited to, silver or gold. Alternatively, a portion of the sample may be associated with a SERS active substrate. Although a semi-quantitative example of the procedure is described in WO2008/019161, we have found that the SERS response of the tags tends to vary such that the results include a significant uncertainty due to non-reproducibility. WO2012/052779 describes an improved method of detecting a taggant quantitatively by use of an internal standard in a particular way. Further improvements in the field are still desirable.